We have described an increase in lung phospholipid content after increasing fetal prolactin levels in the rabbit (J. Clin. Invest. 59:1002, 1977). We have shown, as have others, that human umbilical cord prolactin levels correlate very significantly not only with gestational age, but the incidence of Respiratory Distress Syndrome. At present we are investigating the effect of prolactin on lung maturity as reflected in the mechanical behavior of the lung. The role of prolactin in lung maturation is probably complex and multiple interrelationships exist between various hormones. We are manipulating prolactin production in the experimental animal by pharmacological means (Dopamine, Bromocryptin, Thorazine, etc.) in the human. The relative role of maternal, amniotic and fetal prolactin is being investigated.